1. Field of the Invention
The present invention relates to the field of earth boring tools and more particularly to coring bits utilizing diamond cutting elements.
2. Description of the Prior Art
It is a well established procedure during drilling operations to take a core sample at various levels for both physical and chemical evaluation. Such coring bits are generally characterized by a semi-toroidally shaped drill bit having an inner diameter defining the inner gage with the outer diameter defining the outer gage. In cases where the rock formation being drilled is cohesive or solid, long core samples can be cut and retrieved without difficulty. However, in the case where the rock formation is fractured by a multiplicity of fracture planes or cracks, the grinding action of the rotating drill bit at the bottom of the bore often tends to disturb or mix the core sample as it is cut. As a result, the detailed physical characteristics of the rock formation are lost by the disturbance which is inherently created during the cutting of the core.
Furthermore, as the core is progressively cut, the fracture planes will be shifted as the core moves upwardly within the core barrel. As these planes are shifted or inclined from their original position, each fractured segment of the cut core tends to act like a wedge when displaced from its initial position within the formation. The jumbled wedges bear against the inner surface of the core barrel as the core moves upwardly within the barrel with the result that one or more of the wedges tend to seize and thereby jam the entire core within the barrel. When this occurs, further descent of the coring bit is prevented and all drilling action ceases. Such core jamming can occur at any time and particularly at any point during the coring operation prior to the cutting of the full desired length of the core.
What is needed then is a coring bit, particularly a diamond rotating bit, which has a design which avoids each of these pitfalls of the prior art. In other words, one characteristic of such a design would be a coring bit which does not disturb or which minimally disturbs a core cut from a highly fractured rock formation so that the physical configuration of the cut core within the core barrel is physically identical or substantially identical to the rock formation from which the core was taken. Another characteristic of such an invention would be a coring bit incorporating a design which could be used in a highly fractured formation which left each of the fractured core segments substantially in the same position and orientation as each segment assumed in the original rock formation so that the segments do not become misaligned and wedge against the inner surface of the core barrel, thereby causing a core jam.